Circuits

Three things are needed for an electric current to flow in a circuit:

• the circuit must be complete - electric current can't flow if there are any gaps in the circuit
• a battery or other power source is needed to provide the energy to push the electric current through the wires
• the battery must be connected correctly. One wire must be connected to the (+) positive terminal of the battery and the other wire must be connected to the negative (-) terminal of the battery

We use circuit diagrams to represent electrical circuits. This is the circuit diagram for a simple buzzer:

The battery, switch and buzzer are represented by circuit symbols. You can find out more about circuit diagrams in the under 11 area.

There are two types of electric circuit - series and parallel.

We've already come across series circuits. In this type of circuit, the components are arranged end to end and so the electric current flows through the first component, then through the next component and so on, until it reaches the battery again.

Use Circuit Builder to see what happens if you add more bulbs to a series circuit.

If there's a gap in a series circuit or maybe one of the components has broken, the current can't flow and so, the whole circuit turns off.

In parallel circuits, the components don't have to be end to end because the circuit can have branches. In a parallel circuit, the current splits as it reaches a branch so the current flows around both branches. If there's a gap or broken component in one of the branches of a parallel circuit, the component(s) in other branches will carry on working.

Use Circuit Builder to see what happens if you make a gap in one of the branches of a parallel circuit

If we put a switch in each of the branches of a parallel circuit, we can control each component separately.

This is how our homes are wired; each room has it's own parallel lighting circuit so, if a light bulb blows, the other light bulbs stay lit.

The 13 Amp mains sockets in our homes are also wired in parallel.
We call these parallel circuits ring mains.
The national grid is also a parallel circuit - but on a massive scale!

We can measure the amount of electric current flowing in a circuit with a device called an ammeter. The unit of electric current is the Amp - which is often abbreviated to the letter A, especially if it comes after a number. So, for example, 3 Amps can also be written 3A. To measure the current flowing in a circuit you must connect the ammeter in series with the other components.

You'll find that the current in a series circuit is the same no matter where you put the ammeter. It doesn't get 'used up' as it flows through the components - which is what lots of people think.

What actually happens is, the energy is transferred to the components in the circuit by the electric current which is made up of electric charges. In this case, there is one component, the bulb. The bulb then transfers the energy to the surroundings by heat and light.

Now use Circuit Builder to investigate the electric current flowing in different parts of a parallel circuit.

The current in the branches of a parallel circuit add up to the current flowing in and out of the cell or battery.

If we reduce the thickness of the wires in our circuit, we make it more difficult for the electric current to flow. It's a bit like a pipe carrying water. If we bend the pipe or make it narrower, less water can flow because there's more resistance. By putting pressure on the hose, the resistance increases to the water flowing through the pipe so the water flow is reduced.

Components like motors and bulbs contain very thin wires so if we include them in a circuit, the circuit resistance increases.

Use Circuit Builder to explore what happens if you add more bulbs to a series circuit.

Variable resistance is a bit like having a tap on the hose so the amount of water flowing can easily be adjusted.

If we have thin wires and components like bulbs and motors in our circuit, we increase the circuit resistance and so less current flows. If we want to have greater control over the current flowing round a circuit, we can connect a variable resistor in series with other components.

This is the circuit diagram symbol for a variable resistor:

An electric current won't flow through a circuit unless there's a source of energy like a battery or mains electricity to push the electric charges along through the wire. Voltage is a measure of how much energy the electric charges have between two points in a circuit.

We can measure the energy of electric charges in a circuit before they enter a bulb and after they leave it by putting a voltmeter in parallel across the bulb like this:

The voltmeter measures the energy across the bulb in volts V. In this case, the voltage is 1.5V. Voltmeters are always connected in parallel with the component you want to investigate.

Voltmeters are always connected in parallel with the component you want to investigate.
This is the circuit diagram symbol for a voltmeter:

A fuse is a safety device which stops electric current flowing if an electrical appliance goes wrong and causes the electric current in the circuit to get too big.

Three types of fuses are found in mains plugs: 3A, 5A and 13A. Each fuse contains a special piece of wire which melts once the current reaches the limit. When the wire melts it makes a gap in the circuit and so the current stops flowing.

The mains electricity circuits in our homes are also protected by fuses. These are usually in a fuse box near the electric meter. If there's a problem with an electric appliance the fuse in the plug might 'blow' and sometimes, the fuse in the fuse box blows too.

Some homes have circuit breakers instead of fuses. They do the same job as fuses but they don't rely on melting wires. There are usually quite a few circuit breakers - one for each of the mains electricity circuits in the home.

How much do you know about electrical circuits? Test your knowledge by playing the circuit builder game.